Re: [PATCH] io_uring: drain next sqe instead of shadowing

[Pavel Begunkov <asml.silence@gmail.com>]

> 在 2019/11/21 17:43, Pavel Begunkov 写道:
>> On 11/21/2019 12:26 PM, Jackie Liu wrote:
>>> 2019年11月21日 16:54，Pavel Begunkov <asml.silence@gmail.com> 写道：
>>>>
>>>> If there is a DRAIN in the middle of a link, it uses shadow req. Defer
>>>> the next request/link instead. This:
>>>>
>>>> Pros:
>>>> 1. removes semi-duplicated code
>>>> 2. doesn't allocate memory for shadows
>>>> 3. works better if only the head marked for drain
>>>
>>> I thought about this before, just only drain the head, but if the
>>> latter IO depends
>>> on the link-list, then latter IO will run in front of the link-list.
>>> If we think it
>>> is acceptable, then I think it is ok for me.
>>
>> If I got your point right, latter requests won't run ahead of the
>> link-list. There shouldn't be change of behaviour.
>>
>> The purpose of shadow requests is to mark some request right ahead of
>> the link for draining. This patch uses not a specially added shadow
>> request, but the following regular one. And, as drained IO shouldn't be
>> issued until every request behind completed, this should give the same
>> effect.
>>
>> Am I missed something?
> 
> Thanks for explaining. This is also correct, if I understand
> correctly, It seems that other IOs will wait for all the links are
> done. this is a little different, is it?

Yes, you're right, it also was briefly stated in the patch description
(see Cons). I hope, links + drain in the middle is an uncommon case.
But it can be added back, but may become a little bit uglier.

What do you think, should we care about this case?

> 
> -- 
> Jackie Liu
> 
> 

-- 
Pavel Begunkov